Improve performance

[twoeths]

Improve Performance:

• Add tail implementation, for more information: https://hypirion.com/musings/understanding-persistent-vector-pt-3
• readOnlyForEach is faster than the yield implementation (although the yield looks really nice to me)

Add more convenient methods:

• toTS, readOnlyMap, clone
• more to come: find, filter ...

Benchmark:

• Push (in ms)

num item	upstream	tail impl	immutablejs
100k	53	30	57
1M	420	150	326
10M	4800	1340	1148

• Loop (in ms)

num item	regular array	upstream	tail impl	tail + readOnlyForEach	immutablejs
100k	3	26	30	4	16
1M	14	190	197	10	164
10M	114	2270	2310	60	520

Note that immutablejs statistic is not stable, this records the lowest ones.